This invention relates to boiling water nuclear reactors in general and to fuel bundle assemblies for such reactors in particular.
Boiling Water Reactor (BWR) fuel assemblies include fuel rods and water rods within a flow channel. Water flowing through the many fuel bundles within respective channels provide both coolant and moderator to sustain the nuclear reaction. The moderator function is provided primarily by the higher density liquid. Energy addition along the fuel rods, however, converts some of the water to lower density steam so that its effectiveness as a moderator decreases as the fraction of liquid decreases along the length of the fuel rods. The resultant steam-water flows, referred to as two-phase flows, have higher velocities and cause significant pressure drop along the length of the fuel during typical operation. To reduce this variation in liquid moderator, modem BWR fuel designs include separate flow paths within the fuel bundles which remain filled with liquid (water) over the length of the fuel bundles. These water paths can be configured as one or more round or square tubes (or as cruciform shaped flow passages) generally referred to as water rods. For normal operating conditions, these water rods provide 15% to 20% of the available moderator liquid in BWR fuel bundles.
Current water rod designs provide parallel flow paths through the fuel bundle. They typically include one or more inlet holes near the bottom of the water rods (and the fuel bundle) and one or more outlet holes near the top of the water rods (and the fuel bundle), as illustrated in FIGS. 1 and 2. The placement of these inlet and outlet holes imposes the full bundle pressure drop to drive flow through the water rods. When reactor flow reduces, the pressure difference driving liquid through water rods is also reduced. Current water rods are designed with appropriate inlet flow resistances to maintain very little vapor formation even for such low flow conditions.
A characteristic of such two-phase flow systems is that they can experience flow oscillations under certain conditions of low flow and high power operation. A BWR has a natural tendency to avoid such flow oscillations because the increased steam formed at low flow conditions causes the reactor power to reduce. However, since current water rods are typically designed to remain fill of liquid even at low reactor flow conditions, those water rods limit the natural power reduction at low flow conditions. On the other hand, if the flow restrictions in the water rods are increased to produce significant steam in the water rods at low flow conditions, the current designs result in unfavorable steam in the water rods at normal operation conditions as well.
Two recent water rod designs have been proposed to improve the prior designs mentioned above. One recent design provides varying amounts of steam in the water rod at different reactor flow conditions. This design is intended primarily to enhance spectral shift capability. It utilizes a very small (in cross section) downflow extension tube from the top of the water rod. Thus, water flows upward through a large path and then downward through a small connecting tube for nearly the full length of the fuel before reaching the exit hole. This configuration (illustrated in FIG. 3) has only a short vertical distance between the inlet and outlet holes, with resultant small imposed pressure differential across these holes. For low flow conditions, the downward flow tube is predominantly filled with steam, and the fluid in the upward path is supported like a standpipe with a low pressure differential. The resultant liquid content in the tube is thus quite low, being proportional to the imposed pressure differential. For normal operation, however, the small downflow tube and significant outlet flow restriction combine to severely limit water rod flow. Thus, this design results in significant steam formation in the water rod, with associated unfavorable fuel efficiency, under normal operating conditions.
Another recent water rod design introduces a central standpipe for upflow within a normal water rod design (illustrated in FIG. 4). Using flow restrictions typical of current designs allows for sufficient water rod flow at normal operating conditions to avoid steam formation. For low flow conditions, it was contemplated that the annular region outside of the standpipe would fill with steam when the imposed pressure differential dropped below that necessary to spill liquid over from the top of the standpipe. Unfortunately, analyses have indicated that under such conditions, liquid will flow backward through the upper outlet hole and refill that annular region outside the standpipe (since that region has no bottom drain, it can potentially collect even more liquid than current water rod designs under similar conditions). The only known way to avoid such liquid backflow at the upper outlet hole is to introduce sufficient restriction such that backflow is avoided by the counter flow effects of the escaping vapor. Unfortunately, that amount of flow restriction will again cause unfavorable steam formation in the water rod at normal operating conditions.
The present invention provides a water rod design that is filled with liquid at normal operating conditions but becomes partially filled with steam at low flow conditions. To this end, the water rod(s) are each configured as a siphon tube or siphon water rod (SWR). The newly designed water rods incorporate both an upflow path substantially to the top of the fuel in the fuel rods, followed by a downflow path with outlet holes positioned near the mid-plane of the fuel bundle. This change creates a water rod that operates with a siphon effect, flowing full of liquid at normal operating conditions. This is referred to as the xe2x80x9csiphon modexe2x80x9d of SWR operation. However, at low reactor flow rates, a small amount of vapor formation in the water rod breaks the siphon effect and the water rod transitions to a significant (xcx9c60%) vapor content. This results in the downflow path being virtually filled with vapor, while the upflow path operates as a standpipe with a two-phase mixture. This is referred to as the xe2x80x9cstandpipe modexe2x80x9d of SWR operation.
For some applications, the return or downflow path may be contained within the cross-sectional area of a single water rod. For example, the downflow path could be configured as an internal tube or as an outer annulus as discussed further herein. The siphon water rod may also be configured by interconnecting the tops of two adjacent flow paths. For such an interconnected configuration, it is also possible to incorporate part-length-rods (PLR""s) in the region below the downflow path, as discussed further below.
In all cases, there are two key elements to the disclosed design. First, all of the water rod flow is caused to travel upward from the bottom of the fuel bundle and then downward to the outlet near the fuel bundle mid-plane (i.e., there are no intermediate outlets along the flow path). Second, flow resistances are comparable to current water rod designs such that water rod flows are sufficient to avoid steam content during normal reactor operation(this implies the upflow and downflow portions have similar flow characteristics).
For current water rod designs, water flows are determined by the imposed pressure differential across the inlet and outlet openings minus the fluid density head within the water rod. Thus, current design water rod flow will be zero when the imposed pressure differential just equals the fluid density head in the water rod. For the designs in accordance with this invention, the water rod flow is also driven by the fuel bundle pressure differential imposed between the water rod inlet and outlet openings. However, that imposed pressure differential is significantly lower than current designs (where the outlet is at the top of the water rod) due to the outlet being placed at a much lower elevation in the fuel bundle. However, the net fluid density head working against the flow is also reduced for the siphon water rod, being the net difference between the fluid density heads in the upflow and downflow paths. Thus, when both paths of a siphon water rod are filled with liquid, it is possible to achieve relatively high flows through the water rod even with relatively low imposed pressure differentials. This is the same basic feature utilized for other siphon flow applications (although typical siphon applications make the downflow portion longer than the upflow portion in order to produce net flow even with zero pressure differential between the inlet and outlet openings).
It will thus be appreciated that SWR""s can be designed such that for normal reactor operation, the flow and subcooling entering the water rod are sufficient to maintain liquid water throughout the water rod. Under such circumstances, the siphon water rod provides the same neutron moderation capability as standard water rod designs. However, significant performance difference occurs at reduced reactor flow rates. As the imposed pressure differential decreases, a small amount of steam formation occurs in both current and siphon water rod designs. However, in a siphon water rod such steam formation starts in the downflow path, reducing the fluid density head. That produces a positive feedback by further reducing the water rod flow and causing more steam formation. When sufficient steam formation occurs, the siphon effect is broken and the downflow path quickly fills mostly with steam. Under this condition, the upflow path performs like a standpipe with a density head determined by the imposed pressure differential. Thus, breaking the siphon effect provides the large reduction in water rod liquid content that is needed to improve reactor stability margins at low flow rates.
Accordingly, in its broader aspects, the present invention relates to a fuel bundle assembly for a boiling water nuclear reactor comprising a plurality of fuel rods having respective fuel columns therein, and arranged in an ordered array, extending between upper and lower tie plates, the plurality of fuel rods enclosed within a hollow, open-ended channel member at least partially enclosed by the open-ended channel member; at least one water rod supported on the lower tie plate and extending upwardly toward the upper tie plate, the at least one water rod having an upward flow path including at least one inlet at a lower end of the upward flow path, and a downward flow path including at least one outlet at a lower end of the downward flow path, the at least one outlet located about midway along the fuel columns within the fuel rods.
Other objects and advantages will become apparent from the detailed description which follows.